Method of decolorizing caustic liquors



Patented Jan. 7, 1930 UNITED STATES PATENT OFFICE GEORGE A RICHTER, 0F BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY,

OF BERLIN, NEW HAMPSHIRE, A. CORPORATION OF MAINE METHOD OF DECOLORIZING CAUS'IIC LIQUORS N0 Drawing.

This invention relates to the decolorization of caustic liquors, and more particularly to highly concentrated liquors such as are known on the market as liquid caustic, and consistof aqueous solutions of caustic soda of about more or less, caustic soda concentration. Caustic liquors of this character are likely to have a characteristic cloudy, bluish-gray appearance, as a result of impurities contained therein. These impurities probably either originate in the sodium chloride used as a raw material in the manufacture of the liquor,'or are formed or picked up in the processing apparatus, for instance in the evaporators, pipes, and tanks, and appear in the resulting liquor in a colloidal condition, for example in the form of silicious matter, sodium ferrate, or other colored colloidal compounds. When the liquors are employed in the arts, for example in the rayon -or textile industry, the sodium ferrate or other impurities are objectionable, as they are liable to cause undesirable coloration or otherwise impair the quality-of a product.

It has heretofore been difficult to remove the impurities from caustic liquors. Thus,

textile filter cloths cannot withstand the chemical action of these liquors, While the usual Wire cloths are of insufficiently close weave to permit filtering them out. It is difficult, to separate much of these impurities by settling, as they will not settle, and if their removal is attempted bypassing the liquor through solid filtering mediums such as sand, it is found that the liquor undergoes reaction therewith.

I have discovered that if caustic liquors of the character described are contacted with cellulose fibers in an unwoven, loose or random condition, and consisting substantially only of alpha cellulose, the colored colloidal impurities contained therein are largely re moved without deleteriously affecting the liquor. Undoubtedly the fiber adsorbs the impurities on its surface, and, owing to its inert character undergoes practically no reaction with the liquor other than being Inercerized thereby.

Application filed February 13, 1928. Serial No. 254,166.

fiber consisting substantially only of alpha cellulose, for should the fiber contain nonalpha cellulose components even in amount such as present in ordinary papermaking pulps such as sulphite, it would be quite unsatisfactory for use in decolorizing caustic liquor, for the reason that the ligneous matter, less-resistant celluloses, oxycclluloses, and other non-alpha cellulose components present in sulphite pulp would react with the liquor, not only producing colored reaction products, but consuming caustic soda in the process.

A fiber consisting substantially only of alpha cellulose such as I employed, may be produced, for instance, by refining a wood fiber, for instance,that is, removing its nonalpha cellulose components. For example, a raw pulp such as unbleached'sulphite may be refined by digesting such pulp in caustic soda or other alkaline solution, and then bleaching the digested product. The refining of such pulp may be facilitated by treating the raw pulp with a relatively weak oxiably fiber which has been subjected to alkaline digestion for the removal of waxes, fats, and other impurities which may form colored reaction products with the caustic liquor. The fiber may be used in any suitable condition, it being preferable to beat or fray the fibers so as to increase their adsorptive surface. The beating should not be carried to excess, that is, to a point where the fiber becomes markedly gelatinized, as this tends to depolymerize the cellulose and increase its reactivity with caustic liquor. The present invention will now be described for the purpose of illustration as applied in decolorizing caustic liquor produced by the electrolyticprocess, it being understood that the present If invention is also applicable to caustic liquors produced by other processes. In the manu-- facture of caustic soda by the electrolytic process, it is the practice to electrolyze a solution of sodium chloride in suitable electrolyticcells, to bleed off the dilute caustic soda solution bein formed at the cathode compartments of t e cells and to deliver it into evaporating apparatus, for instance of the multiple-effect type. The caustic soda solution thus formed contains substantial amounts of unelectrolyzed sodium chloride, but when the solution is concentrated in the evaporators, the sodium chloride is precipitated, while the more highly soluble caustic soda remains in solution. If this concentration is carried out to a point Where solid caustic soda is produced, not only is the sodium chloride precipitated, but also colored colloidal impurities such as iron compounds;

but if the concentrating operation is carried out to yield liquid caustic or caustic liquor, such as the present invention deals with, colloidal coloring impurities and fine particles of unelectrolyzed salt are likely to remain in the liquor. In accordance with the present invention, the caustic liquor thus produced is contacted with fiber composed substantially only of alpha cellulose, as previously described. Contact between the fiber and the liquor ma be efiected as by mixing the liquor and the fi er, and then removing the fiber, or

- by passing the liquor through one or more masses ofthe fiber. Thus the liquor may be delivered from the evaporators into agitating tanks, and dry fiber in a shredded or a flufi'ed condition resembling cotton linters added and mixed therewith. Fiber in a suitable flufied condition may, for instance, be produced as by rasping the surface of a relatively thick sheet of such fiber in a form known in the pul industry as drier sheet, for instance,

40 wit wire brushes. About 1% of fiber based on liquor, when intimately mixed with the liquor for a fewminutes, eifects a substantially complete adsorption or removal of the impurities from the liquor. The decolorizing 46 operation may be carried out at room temperature conditions, but if desired the liquor may be delivered from the evaporating apparatus into the agitating tanks while still warm, say, at a temperature of 40 0., or 50 higher, for even under these conditions, other than mercerization thereis practically no reaction between the fiberand the liquor. The fiber suspended in the liquor after the decolorizing operation may be removed as by passing the liquor through a rotary filter designed to prevent passage of the fiber therethrough, the passing of the liquor through the filter being facilitated if the liquor is at elevated temperature, as previously described. As the liquor passes through the filter, .a mass of fiber accumulates on the filter, which serves to catch fine mechanical impurities such as the fine particles of unelectrolyzed salt which may be present in the liquor. A procedure such as described is highly effective in remov- -in soda and kraft pulp manufacture.

liquor with cellulose fiber capa ing impurities from caustic liquors, the resulting liquor being clear and water-white and having an exceedin ly small iron 0011-, tent. In lieu of mixing ber with the liquor and then separating the fiber from the liquor, a removal of the impurities may be eifected by passing the liquor through one or'more masses of fiber, preferably in a relatively loose condition and maintained on a suitable perforated plate or wire cloth support, and designed to permit sutficient time of contact between the liquor and the fiber to effect the desired adsorption and removal of the colloidal coloring im urities from the liquor. In such case, too, t e fiber removes substantially all the impurities resent in the liquor by virtue of its high a sorptivity, much in the same way that coloring impurities are removed from unrefined sugar solutions by passing the solution through bone charcoal. The fiber also serves to catch any mechanical impurities which may be present in the liquor, and other than being mercerized undergoespractically no reactlon with the liquor.

While the process of the present invention is particularly useful in removing impurities from concentrated caustic liquors, it may also be applied to advantage in removing coloring impurities from causic liquors of lower strengths, for example, those employed In such case, the causticized smelt solutions are likely to be more or less turbid, and to contain colored colloidal impurities as a result of the smelting and causticizin operations. These may readily be decolorize by the process of the present invention, to yield clear whiteliquors eminently suitable for the production of high grade ulps.

Having thus descri d certain embodiments of this invention, it is evident that various changes and modifications might be made therein without departing from the spirit or scope of invention as defined in the appended claims.

I claim:

1. A process of decolorizing caustic liquor containing colloidal coloring impurities, which comprises contacting such liquor with fiber consisting substantially only of alpha cellulose, until such fiber has adsorbed sub- .stantially all'the coloring impurities therefrom.

2. A process of decolorizing strong caustic liquor containing colloidal coloring impurities, which comprises uniformly mixing such liquor with fiber consisting substantially only of alpha cellulose, until-such fiber has adsorbed substantially all the coloring impurities therefrom.

3. A process of decolorizing strong caustic liquor containing colloidal coloring impurities, which comprises uniforml mixin such hle of a sorbing the colloidal impurities therefrom substantially without forming or anic reaction products, and removing the her from the decolorized liquor.

4. A rocess of decolorizing causticliquor contaimng colloidal colorin impurities,

which comprises passing such'hquor through.

a mass of'cellulose fiber consistmg substantially only of alpha cellulose.

5. A process of decolorizing caustic'liquor containing colloidal coloring impurities, which comprises mixing such liquor with about 1% by weight of her consistin substantially only of alpha cellulose, an then filtering the liquor ree of fiber through a mass of such fiber. A

6. A rocess of decolorizing caustic liquor containing colloidal coloring impurities, which com rises contacting such liquor with cellulose fiber consisting substantially only of alpha cellulose and which has been beaten to increase its adsorptive surface.

In testimony whereof I have aifixed my signature.

GEORGE A. RICHTER. 

